1. Field of the Invention
The present invention relates to a light-emitting device, particularly to a light-emitting device having high reliability and excellent light extraction efficiency, a manufacturing method thereof, and a lamp.
2. Description of Related Art
In recent years, gallium nitride (GaN)-based compound semiconductor materials, which are nitride-based semiconductors, have become of interest as a semiconductor material for producing a light-emitting device that emits light of short wavelength. A GaN-based compound semiconductor is grown on a substrate of sapphire single crystal, a variety of oxides, or a Group III-V compound, through a metal-organic chemical vapor deposition method (MOCVD method), a molecular-beam epitaxy method (MBE method), or the like.
As a structure of a general GaN-based compound semiconductor light-emitting device, if a sapphire single crystal substrate is employed, an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer are laminated in this order. Since the sapphire substrate is an insulator, the device structure generally has a structure as shown in FIG. 1 in which a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on the n-type semiconductor layer are present in the same plane. Such a GaN-based compound semiconductor light-emitting device has two types: a face up type in which a transparent electrode is used as a positive electrode to extract light from the p-type semiconductor side; and a flip chip type in which a high reflective film of Ag or the like is used as a positive electrode to extract light from the sapphire substrate side.
For providing the transparent electrode on the p-type semiconductor, a metal transparent electrode made of Ni/Au or the like has been conventionally used. In recent years, however, in order to improve the light extraction efficiency of light-emitting devices, translucent conductive oxide films of ITO or the like have been practically and positively used at an industrial level.
External quantum efficiency is used as an index to improve the output from such a light-emitting device. It can be said that a higher external quantum efficiency means a light-emitting device with higher output.
The external quantum efficiency is represented as the multiplication of internal quantum efficiency and light extraction efficiency.
The internal quantum efficiency is the proportion of energy converted into light amongst energy of electrical current injected into the device. Meanwhile, the light extraction efficiency is the proportion of light that can be extracted to the outside amongst light generated inside the semiconductor crystal.
There are mainly two ways to improve the light extraction efficiency. The first is a method of reducing absorption of emission wavelength into an electrode, a protective film, or the like formed on the light extraction surface. The other is a method of reducing reflection loss occurring at an interface between materials having different refractive indexes, such as between a compound semiconductor, an electrode, and a protective film.
Here, one of the reasons why a metal transparent electrode of Ni/Au or the like has been replaced by a translucent conductive oxide film of ITO or the like, is that the absorption of the emission wavelength can be reduced by using the translucent conductive oxide film.
As the method of reducing reflection loss occurring at an interface between materials having different refractive indexes, there is a technique in which the light extraction surface is treated to form an uneven surface. As the treatment method of forming the uneven surface, there has been proposed a light-emitting device in which the compound semiconductor itself is treated to form an uneven surface (for example, Patent Document 1).
However, in the light-emitting device described in Patent Document 1, since the semiconductor material is treated, the semiconductor layer is subjected to loading and is thus damaged. Therefore, although the light extraction efficiency is improved, the internal quantum efficiency is lowered, causing a problem in that the emission intensity can not be increased.
In addition, forming concavo-convex on the translucent conductive oxide film can also improve the light extraction efficiency of the light-emitting device. In this case, the translucent conductive oxide film does not only play a role as a current diffusion layer, but also play a role as a light extraction film.
However, since the refractive index of ITO is as small as 1.9, as compared to 2.6 for the GaN-based compound semiconductor, total reflection occurs at the interface between the ITO and the GaN-based compound semiconductor, which causes a problem of insufficient light extraction.
Titanium oxide has, although depending on its wavelength, a refractive index of 2.6 (wavelength 450 nm), which is approximately the same as that of the GaN-based compound semiconductor. Although titanium oxide is an insulator, it has been recently revealed to become conductive by adding Nb or the like (refer to Non-patent Document 1).
By using such conductive titanium oxide for a transparent electrode and by forming an uneven surface on the titanium oxide rather than forming an uneven surface on the GaN-based compound semiconductor, the light extraction efficiency of the light-emitting device can be readily improved.    [Patent Document 1] Japanese Patent No. 2836687    [Non-patent Document 1] American Institute of Physics, “A Transparent metal: Nb-Doped anatase TiO2”, Applied Physics Letter 86, 252101 (2005) (US), 20 Jun. 2005, p 252101-252103